1. Field of the Invention
The present invention relates to a device and method for protecting an aircraft component from collision with flying objects.
2. Discussion of the Related Art
One of the major hazards to flight safety today is the in-flight impact of birds. Gas turbine engine blading is especially vulnerable to damage. The impact of a bird against a rotating engine blade will create forces for a finite duration of time which are sufficiently high to cause compressive (or shear) failure at the point of contact and also gross structural deformation which may be sufficient to cause failure at some point other than the point of contact.
Although some birds fly above 20,000 feet, and bird strikes have been reported as high as 32,000 feet in the United States and 37,000 feet in Africa, the density of bird traffic decreases exponentially with altitude. Sixty percent of bird strikes occur within a hundred feet of the ground. That figure reflects the crowds of birds in low-altitude flight, and the basic navigational fact that, when startled, birds on the ground have nowhere to go but up, sometimes directly into airplanes that are landing or taking off. Seventy-three percent of bird strikes occur within 500 feet of the ground. For the 27 percent of bird strikes that occur above 500 feet the number of bird strikes declines by 32 percent for every thousand feet of climb. All told, more than 90 percent of bird strikes occur at less than 3,500 feet. There is variation, however, in the outcome of the strikes. Eighty-six percent of those reported cause no damage at all, in part because so many occur just above the ground, where most birds are small, and impact forces are weaker because airplane speeds there are slow.
At the slightly higher altitudes, between 500 and 3,500 feet, the strikes that do occur—some 20 percent of the total—tend to be more dangerous, because airplanes are flying faster and the birds involved are more likely to be large and arrayed in horizontal formation. If you happen to hit one of them, you will likely hit others. At this altitude (500-3,500 feet) the collision closing speed will be a combination of the airplane's 250 miles per hour and an angular component of the geese's own forward speed, which may add 25 miles per hour to the sum.
The industry has come a long way in producing engines that can swallow small birds, and even medium-size ones (such as seagulls; officially, up to 2.5 pounds) without disintegrating or losing significant thrust. The reasons are not difficult to understand. Modern airline engines are hybrids, called turbofans, each of which contains an old-fashioned jet engine in its core, but develops most of its thrust not by shooting a column of high-speed exhaust out the back (as in pure-jet designs), but by reaching forward through itself with a central shaft and driving a propulsion fan. That fan is what you see when you look into the front of an engine. On the engines that power Airbus A320 for example, the fan has a six-foot diameter. It is really just an air pump, similar to an ordinary window fan, but many-bladed, jet-powered, and enormously more forceful. Even when throttled back to minimum speed on the ground, it is capable of sucking in airport workers who stray closer than about six feet to the inlet. More usefully, when it is throttled up to takeoff, climb, or cruise settings, it ingests huge masses of outside air, which it accelerates rearward through the engine casing. At the center of the engine, and just behind the fan, a portion of the accelerated air feeds directly into the jet core, where it is compressed, burned in kerosene-fueled fires, and used to spin turbines (primarily to power the compressors and fan) before being shot as a hot gas out the back. Far more of the fan's accelerated air, however, completely bypasses the jet core and rushes unheated to the rear of the engine, where it returns to the atmosphere. The blown air is known as bypass air. On the A320, it provides as much as 80 percent of the engine thrust.
The fan, in other words, is the ultimate focus of jet-engine design. Its blades overlap, are set at a sharp angle, and are made of strong, flexible, lightweight titanium. These are what birds first hit on the way in, and for the birds the encounter is traumatic.
In any case, turbofan engines are self-protective to some extent, because, when hit by birds, the fan blades may bend without breaking and sling the bird slurry outward, forcing it to blow harmlessly through the bypass ducts, perhaps splattering against protrusions, but never entering the power source—the critical high-speed components that constitute the jet core. Of the 12,028 engines reported to have been struck by birds between 1990 and 2007, two-thirds emerged unscathed from the encounters. Of the remaining third—the engines reported as damaged more than 90 percent continued to produce thrust in some manner, and only 312 were totally destroyed in flight. In short, complete engine failures following bird strikes are rare.
Some, however, will inevitably occur. The reason is that, within the constraints of materials science and practical design, it is simply not yet possible to build turbofan engines that can reliably withstand 250-mile-per-hour collisions with even single birds heavier than the official medium size of 2.5 pounds. In recognition of these realities, certification requirements for the official big-bird test do not require the engine to keep producing thrust, but merely to accommodate its own destruction without running angrily out control, throwing dangerous shrapnel through the engine casing, or bursting fuel lines and catching on fire. Currently, the weight of the big birds used is eight pounds. That is lighter than millions of birds flying around in the North American skies, including typical twelve-pound Canada geese, but it is heavy enough to ensure the death of the (very expensive) test engines. The big-bird tests are single shots, aimed toward the center of the fan, to ensure that parts of the bird are ingested into the critical jet core. Usually a chicken is volunteered for the job. The ASME substitute bird model which accurately reproduces impact loads caused by real birds shows that a 4 kg (8.8 lb) chicken impacting at 112 m/s (275 mph) will produce a 100M Pa (14500 psi) collision shock. The destruction starts when the bird hits the fan. Even as the bird is turning into slurry, it causes fan blades to bend, erode, and fracture-reducing the fan's thrust and sending a hail of titanium debris deeper into the engine. Some of the debris exits harmlessly with the bypass air, but other debris (now mixed with slurry) find its way into the spinning compressors at the entrance to the jet core, where it sets off a cascade of successive failures, with shattered blades and vanes adding to the destructive hail. In response to the disruption, temperatures inside the combustion chambers may rise so high that the debris passing through is turned to molten metal, which splatters against the downstream turbines, even as they themselves are being warped and destroyed by the heat. Needless to say, any part of the bird that has made it this far is vaporized. Meanwhile, overall, the engine will likely be convulsing as it dies.
It is desirable to provide an aircraft gas turbine engine air-inlet bird fragmentation method and apparatus that is aerodynamic and that fragments large birds faster than the closing velocity of their impending collisions in order to safely reduce such birds and other foreign objects to smaller ingestible harmless pieces.